(1) Field of the Invention
The present invention relates to a securing method for inserting a core into a depression portion of a metal member having an open end and connecting the metal member to the core by compressing circumferentially the core and metal member by means of a compression die, and especially relates to a securing method used preferably for a connection between a core and a metal end-fitting in a polymer insulator.
(2) Related Art Statement
Generally, various securing methods for inserting a core into a depression portion of a metal member having an open end and connecting the metal member to the core by compressing circumferentially the core and metal member by means of a compression die have been known. For example, Japanese Patent Publication No.60-54730 discloses, as shown in FIG. 7, such a technique that a core 51 and a compression portion of a metal member 52 are connected by performing a compression operation such that a compression pressure P is circumferentially applied to a compression region L of the core 51 and the metal member 52 to be compressed at once by means of a compression die 53 having a width a corresponding to the compression region L.
In this technique, since the metal member 52 is deformed plastically in a direction vertical to a direction in which the compression pressure is applied by means of the compression die 53, a tensile pressure is applied to the core 51 in its axial direction. Moreover, a compression pressure is applied to the core 51 in its radial direction at the same time.
At the time when use is made of the technique disclosed in Japanese Patent Publication No.60-54730, a sufficient securing strength can be obtained by this technique. However, recently for example in a field of a polymer insulator and so on, there arises a requirement such that a securing method having a more sufficient securing strength is provided so as to produce a polymer insulator having better reliability.
In this respect, the compression pressure may be increased so as to obtain a more sufficient securing strength. However, a predetermined securing strength can not be obtained if a material of the core is varied, and thus there arises a problem such that a securing portion is fractured at an extremely low tensile strength in an actual use.